Lithographic apparatus and substrate edge seal

ABSTRACT

A method of helping to prevent liquid reaching under a substrate is disclosed that includes introducing a gas at a bottom edge of the substrate so that a buffer is created at the edge of the substrate, helping to keep immersion liquid that is present at the top and edge of the substrate away from the bottom surface of the substrate.

FIELD

The present invention relates to a lithographic apparatus. Inparticular, the present invention relates to an immersion system withina lithographic apparatus, wherein a space between an illumination systemand a substrate to be illuminated is filled with a liquid.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, United States patent U.S. Pat. No.4,509,852, hereby incorporated in its entirety by reference) means thatthere is a large body of liquid that must be accelerated during ascanning exposure. This requires additional or more powerful motors andturbulence in the liquid may lead to undesirable and unpredictableeffects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate using a liquidconfinement system (the substrate generally has a larger surface areathan the final element of the projection system). One way which has beenproposed to arrange for this is disclosed in PCT patent application WO99/49504, hereby incorporated in its entirety by reference. Asillustrated in FIGS. 2 and 3, liquid is supplied by at least one inletIN onto the substrate, preferably along the direction of movement of thesubstrate relative to the final element, and is removed by at least oneoutlet OUT after having passed under the projection system. That is, asthe substrate is scanned beneath the element in a −X direction, liquidis supplied at the +X side of the element and taken up at the −X side.FIG. 2 shows the arrangement schematically in which liquid is suppliedvia inlet IN and is taken up on the other side of the element by outletOUT which is connected to a low pressure source. In the illustration ofFIG. 2 the liquid is supplied along the direction of movement of thesubstrate relative to the final element, though this does not need to bethe case. Various orientations and numbers of in-and out-lets positionedaround the final element are possible, one example is illustrated inFIG. 3 in which four sets of an inlet with an outlet on either side areprovided in a regular pattern around the final element.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets IN oneither side of the projection system PL and is removed by a plurality ofdiscrete outlets OUT arranged radially outwardly of the inlets IN. Theinlets IN and OUT can be arranged in a plate with a hole in its centerand through which the projection beam is projected. Liquid is suppliedby one groove inlet IN on one side of the projection system PL andremoved by a plurality of discrete outlets OUT on the other side of theprojection system PL, causing a flow of a thin film of liquid betweenthe projection system PL and the substrate W. The choice of whichcombination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

As depicted in FIG. 5, another solution which has been proposed is toprovide the liquid supply system with a barrier member 12 which extendsalong at least a part of a boundary of the space between the finalelement of the projection system and the substrate table. Liquid 11 issupplied to and/or removed from the space by inlet/outlet 13. Thebarrier member is substantially stationary relative to the projectionsystem PL in the XY plane though there may be some relative movement inthe Z direction (in the direction of the optical axis). A seal 16 isformed between the barrier member and the surface of the substrate. Inan embodiment, the seal is a contactless seal such as a gas seal 14, 15.Such a system with a gas seal is disclosed in U.S. patent applicationpublication no. US 2004-0207824, hereby incorporated in its entirety byreference.

In European patent application publication no. EP 1420300 and UnitedStates patent application publication no. US 2004-0136494, each herebyincorporated in their entirety by reference the idea of a twin or dualstage immersion lithography apparatus is disclosed. Such an apparatus isprovided with two tables for supporting the substrate. Levelingmeasurements are carried out with a table at a first position, withoutimmersion liquid, and exposure is carried out with a table at a secondposition, where immersion liquid is present. Alternatively, theapparatus may have only one table movable between exposure andmeasurement positions.

For one or more reasons, it is desirable to prevent the bottom surfaceof a substrate from coming into contact with an immersion liquid. Afirst reason is that cooling of the substrate edge by cooler liquid orgas may cause temperature variations over the surface of the substrate,possibly causing overlay errors in the pattern printed on the target. Asecond reason is that capillary action of the liquid under the substratemay cause the substrate to stick to the substrate holder. Similarly, ifliquid gets under a closing plate used to seal a space adjacent aprojection system to keep liquid in contact with the projection system,the closing plate may stick to a substrate table used to hold theclosing plate. A third reason is that liquid droplets on the backside ofthe substrate that are still present during a post-exposure bake mayaffect temperature homogeneity and thus may be detrimental to CDcontrol. A fourth reason is that drying stains caused by liquid dropletsdrying on the back of the substrate may create backside particles thatmay contaminate the substrate table, possibly causing a tilt of thesubstrate resulting in overlay errors and focus hotspots. Suchcontamination may instead or also affect substrate clamping pressurebuild-up. Furthermore, there is the risk of cross-contamination of thesubstrate table, a substrate handling system and a substrate temperatureconditioning unit.

SUMMARY

A way to deal with helping to prevent liquid from reaching underneath asubstrate (or other object) is to have one or more liquid outlets (i.e.,low pressure) under a cover ring that surrounds the substrate insubstantially the same plane of the substrate, and another outlet underthe substrate in the substrate holder, just inside the edge of thesubstrate, in order to suck immersion liquid down through these outletsbefore it penetrates too far underneath the substrate. Furthermore, gasmay be supplied under the substrate, which flows towards the outsideedge of the substrate, and joins the immersion liquid in being suckedout of the outlets (such as outlet 20 as shown in FIG. 6). This is knownas a two-phase flow because liquid and gas are both in contact with thesurface of the substrate. As shown in FIG. 6, immersion liquid 11follows the direction of the arrows through a gap between the cover ringCR and the substrate W at pressures of −20000 Pa (i.e. a low pressuresucking at 20000 Pa) in the outlet 24 under the cover ring CR andpressures of, for example, −40000 Pa in the outlet 20 under thesubstrate W. The pressure of the gas under the substrate is atapproximately −40000 Pa and this gas is sucked down the outlet 20 alongwith the immersion liquid 11.

A problem with this arrangement is that immersion liquid 11 penetratesto a relatively large distance underneath the substrate W, possiblycausing localized cooling of the substrate edge and therefore “curlingup” of the substrate edge. Furthermore, two-phase flows may cause atemperature fluctuation because uncontrolled evaporation andcondensation may be allowed to occur. This is largely because liquid andgas have large differences in viscosity and so once an extraction pathis cleared for the gas through the liquid, the liquid is less likely toby cleared along with the gas. This could leave liquid droplets behindon the backside of the substrate, causing one or more of the problemsdiscussed above.

Accordingly, it would be desirable, for example, to provide a systemthat maintains a low pressure under the substrate, keeping the bottomsurface of the substrate dry.

According to an aspect of the invention, there is provided alithographic apparatus, comprising:

a projection system configured to project a patterned radiation beamonto a target portion of a substrate;

a substrate holder configured to hold the substrate;

an outlet configured to provide a low pressure between the substrate andthe substrate holder;

a liquid supply system configured to supply liquid to a space betweenthe projection system and the substrate holder; and

a gas inlet located near a bottom edge of the substrate when held by thesubstrate holder, the gas inlet configured to supply gas to a spacebetween the substrate and the substrate holder so as to substantiallyseal supplied liquid from the low pressure.

Supplying gas to a space between the substrate and the substrate holderat the substrate edge, rather than sucking out liquid that enters thespace, means that a gas buffer may be created near the edge of thesubstrate. This may keep a clear separation between the immersion liquidand a low pressure of a space below the substrate. In addition oralternatively, this may provide a single phase flow under the substrate,possibly reducing energy gradients across this surface. Two-phase flowthat does occur may occurs under, for example, a cover ring rather thanunder the substrate W, thus possibly preventing one or more of theadverse effects described herein.

According to another aspect of the invention, there is provided anapparatus comprising:

a substrate holder configured to hold a substrate,

an outlet configured to provide a low pressure between the substrate andthe substrate holder,

liquid supply system configured to supply liquid to at least the topsurface of the substrate, and

a gas input at the bottom edge of the substrate configured to supply gasto the space between the substrate and the substrate holder so as toform a buffer at the bottom edge of the substrate to keep the liquid outof the low pressure.

According to another aspect of the invention, there is provided alithographic apparatus, comprising:

a substrate table configured to hold a closing plate;

an outlet configured to provide a low pressure between the closing plateand the substrate table;

a liquid supply system configured to supply liquid to a top surface ofthe closing plate; and

a gas inlet located near a bottom edge of the closing plate when held bythe substrate table, the gas inlet configured to supply gas to a spacebetween the closing plate and the substrate table so as to substantiallyseal supplied liquid from the low pressure.

According to another aspect of the invention, there is provided a methodof sealing a low pressure under a substrate from liquid supplied to atleast a top surface of the substrate comprising inputting gas at a loweredge of the substrate to provide a buffer at the edge of the lowpressure through which the liquid may not substantially penetrate.

According to another aspect of the invention, there is provided a devicemanufacturing method, comprising:

projecting, via a liquid, a patterned beam of radiation onto a targetportion of a substrate supported by a substrate holder;

providing a low pressure between the substrate and the substrate holder;and

inputting gas at a lower edge of the substrate to provide a buffer atthe edge of the low pressure through which the liquid may notsubstantially penetrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts another liquid supply system for use in a lithographicprojection apparatus;

FIG. 5 depicts a further liquid supply system for use in a lithographicprojection apparatus;

FIG. 6 depicts a system configured to help prevent liquid from reachingunderneath a substrate;

FIG. 7 depicts a system configured to help prevent liquid from reachingunderneath a substrate (or other object) according to an embodiment ofinvention; and

FIG. 8 depicts a close up view of the meniscus of liquid at the edge ofa substrate.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. UV radiation or DUV radiation);

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

In another mode, the support structure MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As described above, FIG. 6 shows a close up of the space between asubstrate W and a cover ring CR. Immersion liquid 11 enters the gapbetween the cover ring and the substrate and is sucked out of one ormore outlets 24 in the substrate holder WH. The immersion liquid 11penetrates a certain distance under the substrate before being suckeddown via extractor 20, along with gas, which may have been suppliedunderneath the substrate. This causes a two-phase flow under the edge ofthe substrate, which may cause one or more of the problems discussedabove.

Substrate holder WH comprises burls 40 which hold the substrate W at adistance away from the surface of the body of the substrate holder WH.The substrate holder WH may be substantially circular and may thereforecomprise a circumferential ring 30 that has a slightly lower height thanthe burls 40, providing a partial seal. As shown in FIGS. 6 and 7, oneor more further burls may exist at or beyond the edge of the substrateon the substrate holder WH. These may be a few microns higher than thering 30 (but lower in height than burls 40) and may aid inallowing-curl-down of the substrate edge to combat potential curl-upcaused by proximal two-phase flow.

FIG. 7. shows an embodiment of the present invention, whereby, ratherthan an extractor for a mixture of liquid (e.g., water) and gas (e.g.,air), there is in fact a gas inlet 22 through which gas is supplied tocause a gas buffer at the edge of the substrate W. Immersion liquid 11can therefore be prevented from penetrating under substrate W as long asthe pressure of the gas through the inlet 22 is greater than thecapillary pressure of the immersion liquid 11. The immersion liquid 11will then be extracted purely by the extractor 24 under the cover ringCR.

The pressure of the gas input should be carefully regulated. If thepressure is too high, there is a risk of blowing bubbles through theliquid, rather than creating the buffer holding it back. If the pressureis too low, the liquid may not be held back significantly. A key is tocontrol the gas pressure such that the bubble point of the capillary gapbetween the substrate and the substrate holder is not exceeded.

In order to determine the pressure of the input gas through inlet 22,the overpressure must be greater than the capillary pressure of theimmersion liquid (but less than the bubble-forming pressure) which maybe determined as follows:

ΔP=ΔF/area of space under a substrate

$= \frac{f\left( {2\pi\; r} \right)}{\pi\; r^{2}}$

Knowing that

$r = \frac{d}{2}$according to FIG. 8,

$\begin{matrix}{{\Delta\; P} = \frac{f\left( {\pi\; d} \right)}{{\pi\left( \frac{d}{2} \right)}^{2}}} \\{= \frac{4f}{d}}\end{matrix}$

According to FIG. 8,ƒ=σ.cos α

Therefore,

${\Delta\; P} = \frac{4{\sigma cos}\;\alpha}{d}$

The hydrophilic contact angle will be less than 50° and so, for example,with a gap of 20 μm and a contact angle of 30°, the capillary pressurewill be approximately 6500 Pa. Thus, a 7000 Pa overpressure issufficient to overcome this capillary pressure and keep the bottomsurface of the substrate substantially dry, without blowing so hard thatbubbles are produced in the liquid.

The heights of the ring 30 and the burls 40 may be adjusted such that acurl-down of the edges of the substrate W may be allowed. This may helpto compensate for any curl-up caused by the cooling action of atwo-phase flow of the immersion liquid 11 and the gas through inlet 22.One way of doing this is by having the burls 40 about 3 or 4 μm higherin height than the ring 30. The edge of the substrate W is thereby lesssupported and atmospheric or immersion liquid pressure will have agreater effect on the unsupported edge of the substrate.

A ring 30 may be located radially inward from the gas inlet 22 andradially outward of the burls 40. Additionally or alternatively, one ormore rings, like ring 30, may be located radially outward from the gasinlet 22 and radially inward from the extractor 24.

The supplied gas should not only have its pressure carefully controlled,but also its temperature. The temperature of the overpressurized gasshould be as close to the temperature of the substrate and the substrateholder as possible.

The entire apparatus and method hereinbefore described is also suitablefor preventing liquid from penetrating under other objects. For example,it may suitable for preventing liquid from penetrating under a closingplate when, for example, substrates are being swapped between exposures.In the figures, the closing plate would take the place of the substrateand the space between either the substrate table and the closing plateis provided with a gas input in the same way. The substrate table isalso affected by cooling-related distortions, which in turn may causeoverlay errors on a substrates placed on the substrate table. Thesedistortions may also cause an error in focusing and imaging. Thesubstrate table should therefore also be kept at a substantiallyconstant temperature.

Furthermore, a closing plate should not stick to a substrate table, e.g.by capillary action of the liquid seeping under the plate. This mayaffect the reliability of the closing plate and could cause a localcurl-up of the edge of the plate, which could affect the build-up of aclamping pressure that attaches the closing plate properly to thesubstrate table or to a liquid supply system structure. The apparatus istherefore useful for keeping a gas space between the closing plate andthe substrate table. Also, when moving the substrate table relative tothe liquid supply system, this apparatus and method is useful forpreventing spillage of the liquid during the moving.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate-used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The term “ring” herein should be understood as including other shapesthan circular.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

One or more embodiments of the invention may be applied to any immersionlithography apparatus, in particular, but not exclusively, those typesmentioned above and whether the immersion liquid is provided in the formof a bath or only on a localized surface area of the substrate. A liquidsupply system as contemplated herein should be broadly construed. Incertain embodiments, it may be a mechanism or combination of structuresthat provides a liquid to a space between the projection system and thesubstrate and/or substrate table. It may comprise a combination of oneor more structures, one or more liquid inlets, one or more gas inlets,one or more gas outlets, and/or one or more liquid outlets that provideliquid to the space. In an embodiment, a surface of the space may be aportion of the substrate and/or substrate table, or a surface of thespace may completely cover a surface of the substrate and/or substratetable, or the space may envelop the substrate and/or substrate table.The liquid supply system may optionally further include one or moreelements to control the position, quantity, quality, shape, flow rate orany other features of the liquid.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus, comprising: a projection system configuredto project a patterned radiation beam onto a target portion of asubstrate; a substrate holder configured to hold the substrate; anoutlet configured to provide a low pressure between the substrate andthe substrate holder; a liquid supply system configured to supply liquidto a space between the projection system and the substrate holder; and agas inlet located near a bottom edge of the substrate when held by thesubstrate holder, the gas inlet configured to supply gas to a spacebetween the substrate and the substrate holder so as to substantiallyseal supplied liquid from the low pressure, wherein the gas tosubstantially seal supplied liquid from the low pressure is notsubstantially removed radially outward, relative to the center of thesubstrate, of the gas inlet.
 2. The apparatus according to claim 1,wherein the liquid, when supplied, surrounds all sides of the substrateexcept for a bottom surface of the substrate.
 3. The apparatus accordingto claim 1, further comprising an extractor configured to extract liquidrepelled by the seal.
 4. The apparatus according to claim 3, wherein theextractor is in a same surface as the gas inlet but outside of an areacovered by the substrate when the substrate is held by the substrateholder.
 5. The apparatus according to claim 1, wherein an input pressureof the gas, when supplied, is greater than or equal to a capillarypressure of the liquid when between the substrate and the substrateholder.
 6. The apparatus according to claim 5, wherein the inputpressure is an overpressure of about 7000 Pa and a distance between thesubstrate and the substrate holder is about 20 μm.
 7. The apparatusaccording to claim 1, wherein the substrate holder comprises acircumferential ring and burls, the ring configured to facilitate a sealunder the substrate when held by the substrate holder and the burlsconfigured to support the substrate.
 8. The apparatus according to claim7, wherein the burls are higher than the ring, causing a gap to existbetween the ring and the substrate when held by the substrate holder andallowing curl-down of the edge of the substrate.
 9. The apparatusaccording to claim 8, wherein the gap between the ring and the substrateis 3 to 4 μm.
 10. The apparatus according to claim 8, wherein the gasinlet is located inside of the ring but outside the burls.
 11. Theapparatus according to claim 8, further comprising a secondcircumferential ring concentrically inside the first circumferentialring and the gas inlet is between the first and second rings.
 12. Theapparatus according to claim 1, further comprising a temperature controldevice configured to control a temperature of supplied gas.
 13. Alithographic apparatus, comprising: a substrate table configured to holda closing plate; an outlet configured to provide a low pressure betweenthe closing plate and the substrate table; a liquid supply systemconfigured to supply liquid to a top surface of the closing plate; and agas inlet located near a bottom edge of the closing plate when held bythe substrate table, the gas inlet configured to supply gas to a spacebetween the closing plate and the substrate table so as to substantiallyseal supplied liquid from the low pressure, wherein the gas tosubstantially seal supplied liquid from the low pressure is notsubstantially removed radially outward, relative to the center of theclosing plate, of the gas inlet.
 14. The apparatus according to claim 1,further comprising an extractor configured to extract liquid repelled bythe seal.
 15. A method of sealing a low pressure under a substrate fromliquid supplied to at least a top surface of the substrate comprisinginputting gas at a lower edge of the substrate to provide a buffer atthe edge of the low pressure through which the liquid may notsubstantially penetrate, wherein the gas provided as a buffer throughwhich the liquid may not substantially penetrate is not substantiallyremoved at a location radially outward, relative to the center of thesubstrate, of the inputting of the gas.
 16. The method according toclaim 15, further comprising extracting liquid repelled by the buffer.17. The method according to claim 15, wherein an input pressure of thegas is greater than or equal to a capillary pressure of the liquidbetween the substrate and the substrate holder.
 18. A devicemanufacturing method, comprising: projecting, via a liquid, a patternedbeam of radiation onto a target portion of a substrate supported by asubstrate holder; providing a low pressure between the substrate and thesubstrate holder; and inputting gas at a lower edge of the substrate toprovide a buffer at the edge of the low pressure through which theliquid may not substantially penetrate, wherein the gas provided as abuffer through which the liquid may not substantially penetrate is notsubstantially removed at a location radially outward, relative to thecenter of the substrate, of the inputting of the gas.
 19. The methodaccording to claim 18, further comprising extracting the liquid outsideof the lower edge of the substrate to remove liquid repelled by thebuffer.
 20. The method according to claim 18, wherein an input pressureof the gas is greater than or equal to a capillary pressure of theliquid between the substrate and the substrate holder.
 21. The apparatusaccording to claim 1, configured control the pressure of the gas tosubstantially seal supplied liquid from the low pressure such that abubble point of the space between the substrate and the substrate holderis not exceeded.
 22. The apparatus according to claim 13, configuredcontrol the pressure of the gas to substantially seal supplied liquidfrom the low pressure such that a bubble point of the space between theclosing plate and the substrate table is not exceeded.
 23. The methodaccording to claim 15, further comprising controlling the pressure ofthe gas provided as a buffer such that a bubble point of a gap betweenthe substrate and a structure adjacent the substrate is not exceeded.24. The method according to claim 18, further comprising controlling thepressure of the gas provided as a buffer such that a bubble point of agap between the substrate and a structure adjacent the substrate is notexceeded.